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Various High-speed Engines.

pressure cylinder in order. The high-pressure pistons further
act as valves for similarly distributing steam to the low-pressure
cylinders. Engine D has a special valve of annular form, through
which the steam passes both to and from the cylinders, as shewn
by arrows, c and D are single-acting engines, so far as each

of

627.

cylinder is concerned, the steam pressure being felt only on one
side of the piston ; but, taking the three cylinders together, there
is an impulse every third of a revolution, instead of every half
revolution, as in ordinary single-cylinder double-acting engines.
In both engines it is only necessary to tura on steam to start in
any position, while if reversal is required, an extra four-way plug-
cock, called a reversing valve, is interposed, whose duty is to